1 and 1/2, or 3/2 kg 1500g is 1.5kg
Because it is! There is no proper answer. It's the same as asking why and inch is bigger than a centimetre.
They are equivalent.
2 kg is bigger
No, 2 Kg (Kilograms) is greater than 20g (grams).
No, 3 is 1 unit bigger than 2
Really the question boils down to "which mass is greater - 2 kg or 3 kg". Obviously 3 kg is more than 2 kg no matter what it is composed of. Although the pillow may be bigger, it has less total mass - which is being asked about here.